The Celtics didn’t maintain that effort, though, and suffered their fifth consecutive defeat, 104-93, dropping their record to 4-9 entering Friday’s game at TD Garden against the 10-1 Indiana Pacers.

The first half was crucial for the Celtics, if only because it was a glimpse of what their defense can achieve when they walk the walk, doing all the things they say they must to do every night to have a chance.

“First half, that’s how we want to play,” forward Jared Sullinger said. “Now we’ve just got to put 48 minutes together and we’ll come out with a win.”

The Celtics haven’t been great on either end this season, but their offense figures to be limited — they rank 26th in scoring offense (93.4 points per game) — simply because they lack the overall firepower to pile up points on a consistent basis.

So, defense must be the Celtics’ hallmark for them to remain competitive, and it’s no surprise that coach Brad Stevens has preached defense from Day 1. It remains his biggest emphasis — and it remains an area where the Celtics have been inconsistent.

“Right now, transition and the glass are the things that are killing us,” Stevens said. “If we can curb those two things, we’ll be in pretty good shape. The rest of it becomes probably overanalyzed because it’s really those two things that are hurting us the most.”

The Celtics have allowed an average of 13.1 fast-break points per game, which ranks them in the top half of the league in transition defense. But they are 28th when it comes to second-chance points, as they surrender an average of 15.7 per game.

One factor that contributes is that the Celtics also rank 28th in defensive rebound percentage, grabbing only 71.9 percent of the available defensive rebounds in a game.

But even if the Celtics had more scorers, Stevens would still be a defense-first coach, as he has always been. He pointed to teams such as the Pacers, Spurs, and Miami Heat, noting how each of them can shut down opposing teams.

“You have to have a defensive DNA,” Stevens said. “That wins teams championships, above and beyond just the obvious good players.”

From the outside, it’s easy to look at the Celtics’ defensive inconsistencies and point to the fact that they have a new coach teaching a new system to a group of mostly new players.

Through 13 games, Stevens said, he believes that the players have a decent grasp of certain concepts and that it will get better over time.

Sullinger agreed.

“We know what we need to do,” Sullinger said. “Sometimes we get caught up in, as a team individually, worrying about our guys instead of worrying about the ball. Once we figure everything out, I think we’re going to be pretty good.”

Alongside Stevens is assistant coach Ron Adams, one of the top defensive minds in basketball.

“There’s no secret formula,” Adams said. “There’s no scheme that just all of the sudden shuts people down. It’s the energy. It’s the energy and the fundamentals and the focus to execute the schemes, with, I underscore, the energy and the intensity to execute the scheme.

“It’s funny, at this level, you teach stuff and then guys, all of a sudden, it’s not working and they want to look at the scheme. No, you have to look at your individual intensity and your collective intensity on executing scheme.”

To that point, Stevens said there are little things that the Celtics aren’t doing, such as not finishing plays or getting back on defense quickly enough.

To Gerald Wallace, defense isn’t a difficult concept.

“Defense is defense,” he said. “It’s guarding your yard and a half, as they say.”

Aside from Wednesday, Wallace said the only other time the Celtics truly played well on defense this season was at Miami Nov. 9, when they won on a buzzer-beater.

Wallace said the Celtics kept the Heat out of transition and off the free throw line, and forced them away from their tendencies.

“We did a great job of contesting in that game and taking them out of what they do so well,” Wallace said. “That’s the defense that we’ve got to play as a team to be a winning team.”